Antibiotic resistance rates among hospital-acquired bacteria have reached crisis proportions, particularly in health care facilities in the northeastern United States. In particular, two pathogens, Klebsiella pneumoniae and Acinetobacter baumannii, have developed resistance to all commonly-used antibiotics. Isolates resistant to all beta-lactams (including carbapenems), fluoroquinolones, and aminoglycosides have been documented. Clinicians often resort to the use of polymyxin antibiotics for therapy of these multidrug-resistant bacteria, however toxicity concerns surround these agents, and polymyxin-resistant strains have been reported. For patients with infections with these multidrug-resistant pathogens, no known effective therapy exists. This study will analyze three molecular mechanisms contributing to multidrug resistance in K. pneumoniae and A. baumannii: 1) beta-lactamases, 2) porin deficiencies, and 3) antibiotic efflux systems. Using real time reverse transcriptase PCR, expression of genes encoding beta-lactamases, outer membrane porins, and efflux systems will be measured in highly-resistant strains and their expression will be compared to that found in susceptible isolates. In addition, purported regulatory genes will be characterized by DNA sequencing, to determine if specific mutations affect the genetic control of many of these resistance genes. Ultimately, understanding how these bacteria become so resistant to antibiotics will be important for the development of novel antibacterial agents. For example, the development of novel beta-lactamase inhibitors or efflux system inhibitors may prove useful against some of these isolates. [unreadable] [unreadable] The inter- and intra- hospital transmission of strains of multidrug-resistant K. pneumoniae and A. baumannii has become a serious public health issue. Despite heightened awareness and infection control efforts, the emergence and spread of these pathogens has been relentless. The clinical and economic consequences of nosocomial infections due to multidrug-resistant bacteria are staggering. [unreadable] [unreadable] [unreadable]